Flash Guide Number Calculator
Fill in the values and the answer updates as you type.
How the Flash Guide Number Calculator works
Guide number = distance × f-number at ISO 100, so aperture = GN ÷ distance. Raising ISO by 4× doubles the effective guide number. Manufacturers quote GN for a specific zoom position, so a wider zoom setting reduces it. Bouncing the flash off a ceiling loses a stop or more, because the light travels farther. Example: A flash with GN 40 at 5 m needs f/8 at ISO 100.
Inputs and results
The Flash Guide Number Calculator takes 3 inputs and gives 3 results.
What you enter
- Guide number (ISO 100, metres) — starts at 40.
- Flash to subject distance (m) — starts at 5.
- ISO — starts at 100.
What you get
- Aperture needed (f-number)
- Guide number at your ISO
- Maximum distance at f/2.8 (m)
Example: Flash Guide Number Calculator with the starting values
With guide number (iso 100, metres) 40, flash to subject distance 5 m and iso 100, the calculator returns:
| Aperture needed (f-number) | 8 |
|---|---|
| Guide number at your ISO | 40 |
| Maximum distance at f/2.8 (m) | 14.3 |
Change any value above and the results update straight away.
Flash Guide Number Calculator FAQ
How do I use the Flash Guide Number Calculator?
Enter guide number (iso 100, metres), flash to subject distance and iso in the fields, then read aperture needed (f-number), guide number at your iso and maximum distance at f/2.8 (m) in the result panel. The answer updates as you type, and you can press Calculate or Enter to refresh it. Use “Copy link” to share your exact numbers with someone else.
What does the Flash Guide Number Calculator calculate?
Aperture needed for correct flash exposure at a given distance. It returns aperture needed (f-number), guide number at your iso and maximum distance at f/2.8 (m) from the 3 values you provide.
What values does the Flash Guide Number Calculator start with?
It opens with guide number (iso 100, metres) 40, flash to subject distance 5 m and iso 100 so you can see a working result straight away. Replace them with your own numbers, or press Reset to return to these starting values.
Is the Flash Guide Number Calculator free and private?
Yes. It is free, needs no account, and runs in your browser, so nothing you type is sent to or stored on our servers. See the Privacy Policy for details.
Can I rely on the Flash Guide Number Calculator for important decisions?
Treat the result as a well-rounded estimate. Real situations often have details a general calculator cannot know, so confirm important decisions with a qualified professional. See the Disclaimer for more.